Softbank has been luring users away from rivals with an aggressive price-cutting strategy since it bought Vodafone Group's (VOD.L: Quote, Profile, Research) Japan mobile unit in 2006. It expanded its customer base by nearly a fifth last business year.

That forced DoCoMo, which holds about half the market, and KDDI to offer discounts as well, fuelling a price war in a mature market where the number of subscribers has reached 107 million people, or about 85 percent of Japan's population.

In March, after KDDI’s Tu-Ka brand was stopped, the company observed a decrease in its subscriber number but the Au brand added 115,400 new customers last month.